Greenstone is not a specific mineral but a descriptive term for a particular appearance found within various types of rock. The name refers to the characteristic green coloration resulting from the presence of minerals like chlorite, epidote, or actinolite. Geologically, this term applies to metamorphosed volcanic rocks, specifically those that underwent low to moderate temperature changes, and to certain sedimentary deposits where green minerals are prevalent. Understanding what type of rock is greenstone requires looking at its formation, composition, and the geological settings where it originates.
Defining Greenstone: A Visual Descriptor
The most immediate way to identify greenstone is through its visual properties. The dominant color is typically a shade of green, ranging from light olive to dark forest or bluish-green. This hue is almost always caused by specific minerals, primarily chlorite and epidote, though glauconite or actinolite can also be responsible. While the color is the most obvious feature, geologists look beyond the surface to examine the texture and the original rock material, known as the protolith, to classify the stone accurately.
Metamorphic Origin: The Archean Connection
A significant portion of what geologists classify as greenstone is metamorphosed basalt. When ancient volcanic rocks, formed on the ocean floor billions of years ago, are subjected to heat and pressure but not enough to melt them completely, they transform. This process turns the original basalt into a metamorphic rock that often displays the green hues associated with the term. These greenstone belts are geological treasures, particularly in the Archean eon, as they provide some of the oldest known pieces of the Earth's crust, locking in evidence of early planetary conditions.
Volcanic Protolith
The starting material for much metamorphic greenstone is mafic volcanic rock. This means the original rock was rich in magnesium and iron before the metamorphism occurred. The intense but controlled pressure and temperature during the metamorphic event recrystallize the minerals, creating the dense, fine-grained appearance characteristic of these stones. The resulting rock is hard and durable, making it suitable for various applications throughout human history.
Sedimentary Greenstone: Glauconitic Sandstone
Not all green-hued stones are the result of intense heat and pressure. In sedimentary geology, greenstone can refer to sandstone that is rich in the mineral glauconite. Glauconite is a greenish mineral that forms in marine environments, particularly in shallow seas. When this mineral is abundant, it gives the sandstone a distinct green color, leading to the classification of glauconitic sandstone as a type of greenstone. This formation process is fundamentally different from the metamorphic variety, as it involves deposition and cementation rather than recrystallization.
Mineral Composition and Hardness
The specific mineral content dictates the exact shade and durability of the stone. Chlorite-rich varieties are common in metamorphic rocks, providing a soft to medium-hard material. Epidote-bearing rocks tend to be tougher and display a darker green. The Mohs hardness of these rocks typically falls in the range of 5 to 7, depending on the specific mineral mix. This level of hardness makes them resistant to scratching, which is why they have been historically used for tools, building stones, and decorative purposes.
Historical and Industrial Applications
Humanity has interacted with greenstone for millennia, long before modern geological classification. Indigenous cultures, particularly in regions like New Zealand, carved intricate tools and ceremonial weapons from hard greenstone rocks like nephrite jade. In construction, greenstone or rocks containing green minerals have been used as dimension stone for buildings and monuments. While not as famous as granite or marble, its unique color and historical significance give it a distinct value in architecture and art.
